This is the Frobenius coin problem for n=2.

The general idea is that whenever gcd(3,5)=1 you can go on forever eventually.

This formula in this case is,

(x-1)(y-1)=(5-1)(3-2)=8

Thus, 8 is the smallest number from where everything is obtainable, or you can think of it as 7 is the largest number from which no combination is possible.